snapuserd: Add XorSink

We can compute the xor while decompessing by computing it within a
buffer. This would allow us to use a smaller buffer than the full block
size if we wish to cut down on the extra memory required.

Bug: 177104308
Test: cow_snapuserd_test (Snapuserd_Test.xor_buffer)
Change-Id: Id18505841c77340760cea2d20e83454f1142b9b5